1. Technical Field
The present invention relates to a linear type direct drive motor or a rotary type direct drive motor, which can be used to table of a machine tool. The present invention relates to a technique capable of preventing the output torque from decreasing when a mounting error occurs between a motor and a position detector.
2. Related Art
Direct drive motors are usable to realize high-speed and high-accurate positioning of the table of a machine tool when a motor is used to drive a table directly without using any ball screw or any speed reduction device.
When the motor is a direct drive type, the positioning accuracy of the motor directly influences the positioning accuracy of the table because of non-presence of a speed reduction mechanism. Therefore, the direct drive motor is required to be accurate enough in positioning. In general, the direct drive motor requires a high-resolution position detector to detect the position of its table (movable element of the direct drive motor) when the motor operates.
In general, the linear direct drive motor is composed of a movable element (i.e., a moving body) and a stator fixed to a bed. On the other hand, the rotary direct drive motor is composed of a rotor (i.e., a moving body) and a stator. The present invention is applicable not only to the linear drive motor but also to the rotary drive motor. In the following description, the terminology “movable element” includes various types of moving bodies, including the above-described rotor of the rotary direct drive motor.
FIG. 8 is a block diagram illustrating a circuit configuration of a control system for the above-described direct drive motor. The control system for a direct drive motor 11 includes a position detector 12, two proportional amplifiers 21 and 22, a current distributor 23, an integrating amplifier 24, a current control unit 25, a differentiator 26, a three-phase PWM inverter 28, and a current detector 29.
In the control system illustrated in FIG. 8, if a position command θ* is input, the proportional amplifier 21 amplifies a difference between an input command value of the position command θ* and a detection value obtained by the position detector 12 (i.e., the position of a movable element in the direct drive motor 11). The proportional amplifier 21 outputs the amplified difference as a speed command V* for the movable element.
Then, the proportional amplifier 22 and the integrating amplifier 24 cooperatively perform PI operation on a difference between the speed command V* and the speed of the movable element to generate a torque command T*.
The differentiator 26 can obtain the speed of the movable element by differentiating the detection value obtained by the position detector 12. The current distributor 23 receives the torque command T* and generates two of three-phase current commands Iu*, Iv*, and Iw* (i.e., current commands Iu* and Iv*). The current distributor 23 outputs the generated current commands Iu* and Iv* to the current control unit 25. In this case, in generating the current commands, the current distributor 23 takes the detection value supplied from the position detector 12 into consideration.
The current control unit 25 generates three-phase voltage commands eu*, ev*, and ew* based on the current commands Iu* and Iv* received from the current distributor 23 as well as based on a current command Iw* that can be derived from a formula representing the relationship iu*+iv*+iw*=0. The current control unit 25 outputs the generated three-phase voltage commands eu*, ev*, and ew* to the three-phase PWM inverter 28.
The three-phase PWM inverter 28 converts a direct current (DC) voltage supplied from a DC power source 27 into three-phase alternating current (AC) voltage components based on the three-phase voltage commands eu*, ev*, and ew*. The direct drive motor 11 can be driven when the three-phase AC voltage components are applied from the three-phase PWM inverter 28.
The voltage components actually applied to the direct drive motor 11 are three-phase voltage commands eu*, ev*, and ew* that the current control unit 25 can obtain with reference to differences relative to current detection values iu, iv, and iw detected by the current detector 29.
FIG. 9 illustrates thrust/torque characteristics obtainable when the current phase slides in a state where the movable element of the direct drive motor 11 is fixed. It is understood from FIG. 9 that if the current remains the same the thrust/torque of the direct drive motor 11 can be maximized when the current phase is controlled to be 90°.
To generate the thrust/torque of the direct drive motor 11 efficiently, it is required to control the phase of current supplied to a stator coil of the direct drive motor 11 to have a predetermined phase difference relative to the magnetic pole position of the movable element.
Therefore, a relative positional relationship between the actual position of the movable element and the position detection value detected by the position detector 12 is required to be identical to a predetermined positional relationship having been set beforehand in a control circuit.
However, it is usual that a mounting error occurs between the direct drive motor 11 and the position detector 12 due to clearances provided for respective mounting holes and female screw holes, or as a result of mechanical errors in machining, such as positional deviations between the mounting holes and the female screw holes. If the mounting error occurs, the current phase θ1 becomes equal to 90°+an error component (electrical angle). As a result, the output torque decreases.
Further, the direct drive motor 11 employs a multi-polar structure that can improve the positioning accuracy of the motor. Employing the multi-polar structure is advantageous in that a motor control angle relative to a motor moving distance can be increased. However, the multi-polar motor is disadvantageous in that the torque decreases greatly when the mounting error occurs. More specifically, it is now assumed that a rotary type motor is equipped with n pole pairs and has a relationship θ″=nθ′, in which θ′ represents a mechanical angle and θ″ represents an electrical angle.
For example, if a mounting error is equivalent to +1° in terms of the mechanical angle, a motor equipped with four pole pairs has an error amount of +4° in terms of the electrical angle. A motor equipped with 32 pole pairs has an error amount of +32° in terms of the electrical angle.
In this case, as illustrated in FIG. 9, the output torque decreases to a 99.8% level (=0.2% torque reduction) in the former case and to a 85% level (=15% torque reduction) in the latter case. It is understood that the torque of the multi-polar motor greatly decreases if the magnitude of the mounting error becomes larger.
Therefore, if the direct drive motor employs the above-described multi-polar structure, the direct drive motor 11 and the position detector 12 are required to be positioned accurately. However, to assure machining accuracy and ensure assembling, the clearances provided for respective mounting holes and female screw holes of the direct drive motor 11 and the position detector 12 cannot be omitted.
Therefore, as discussed in JP 2000-166278 A, electrically correcting a mounting error after completing the assembling is conventionally known as a magnetic pole position correction method.
However, the magnetic pole position correction method discussed in JP 2000-166278 A is directed to the electrical correction of amounting error between a direct drive motor and a position detector. The method includes calculating a magnetic pole position correction value based on an output of a d-axis current error amplifier while causing the rotation axis of the motor to rotate at a constant speed. Therefore, the magnetic pole position correction method discussed in JP 2000-166278 A is not applicable to a rotation axis whose movable angle is limited and a linear axis whose movable distance is limited.
Further, at an initial state, the positional relationship between the direct drive motor and the position detector may greatly deviate from an optimum magnetic pole position correction value due to a mistake or an error in a mounting operation of the direct drive motor and the position detector. For example, in a state where the above-described magnetic pole position correction is not yet performed, the angular deviation amount may exceed the range of ±90° in terms of the electrical angle.
In this case, the terminology “mistake in the mounting operation” indicates that a movable element or a stator of the motor is erroneously attached to at an angular position different or deviated from a normal angle.
Further, the terminology “mounting error” is amounting angle error that may be caused due to the clearances provided for respective mounting holes and female screw holes, or by mechanical errors in machining, such as positional deviations between the mounting holes and the female screw holes. If the deviation in the positional relationship between the direct drive motor and the position detector in the mounted state exceeds the limit of the ±90° range in terms of the electrical angle, the direct drive motor becomes uncontrollable. In such a situation, the positioning of the motor will fail and the magnetic pole position correction cannot be performed. Further, the table may move abnormally beyond the limit of an estimated movable range and may collide with a neighboring machine component and may ultimately be damaged.
The present invention solves the above-described problems. To this end, the present invention has an object to provide a method applicable to a direct drive motor that is employed for a table. According to the present invention, a control system can correct amounting error between the direct drive motor and a position detector.
Further, another object of the present invention is to provide a method capable of realizing a safe correction if a mounting error occurs between a direct drive motor and a position detector in a case where the direct drive motor is employed for a table.